This invention relates to a system and method for maintaining a substantially constant tension on a material being unwound from or wound onto a roll of material. Specifically, the present invention relates to a control system which constantly updates or adjusts the speed of rotation of a shaft on which the roll of material is mounted to maintain substantially constant predetermined tension on the material.
In many situations, it is important that material being processed or handled remain at a substantially constant tension to avoid stretching or damaging the material. One such situation in which maintaining material at a substantially constant tension is critical is during the processing or manufacture of filtering material used to make filters for filtering various types of fluids. One illustrative example of such filters are filters used for filtering etchant solutions used to make integrated circuits.
The etchant solutions used in integrated circuit fabrication are filtered to remove impurities or contaminants from the etchant solutions. Any impurities or contaminants which remain in the etchant solutions after filtering can cause the integrated circuit to be flawed. Therefore, the filters used to filter the etchant solutions are vitally important to the success of manufacturing the integrated circuits, especially as the size of the line widths on the integrated circuits decrease. Impurities or contaminants in the etchant solutions can block lines on an integrated circuit chip, thereby causing the chip to be flawed.
Delicate materials are used to make filters such as the filters used for filtering etchant solutions. The filters are designed to have a predetermined pore size to remove impurities and contaminants larger than the predetermined pore size from the etchant solutions passing through the filters. The material for making the filters is typically a fine nylon or teflon mesh material. Prior to manufacturing the filters, a film material is pressed onto the nylon or teflon mesh to provide a backing on the mesh. Illustratively, the film material is a mylar film.
During processing, filter materials are wound and unwound from rolls of material at relatively low speeds. It is necessary to keep the tension of the material substantially constant while winding and unwinding the material on these rolls. If the tension on the material rises above a predetermined level the pore size of the filter material can be stretched. By stretching the material, the pore size of the filter material is increased. The stretched filter material permits contaminants or impurities to remain in the etchant solution that would otherwise be removed by unstretched filter material. Therefore, when handling the material, the material must be maintained at a substantially constant predetermined tension while being wound onto or unwound from the roll of material to prevent stretching of the material.
It is known to provide a "dancing arm" system for maintaining the tension of a material being unwound from or wound onto a roll substantially constant. The dancing arm system uses a roller which contacts the material at a location spaced apart from main roll of material. The roller is forced against the material with a force related to the desired predetermined tension. The weight of the roller causes problems as the desired tension on the material decreases. The present invention provides several advantages over the dancing arm system. The present system is more compact than the dancing arm system. The present invention is also able to measure and maintain smaller tensions on the material more accurately than the dancing arm system. In addition, the present system is a non-invasive system which does not contact the material. This is an important advantage over the dancing arm system, especially when handling delicate materials which could be damaged by the roller of the dancing arm system.
One object of the present invention is to provide a device for winding or unwinding a roll of material which is capable of maintaining the tension of the material being wound onto or unwound from the roll of material at a substantially constant tension.
According to the present invention, a control system is provided for maintaining a substantially constant predetermined tension on material being unwound from or wound onto a roll of material. The control system includes a frame, a shaft for holding the roll of material, and a motor coupled to the shaft for rotating the shaft at a predetermined speed to maintain the material at a substantially constant predetermined tension. The control system also includes means for rotatably coupling the shaft and the motor to the frame to position the motor in an initial preset position relative to the frame. The coupling means permits limited rotational movement of the motor relative to the frame in response to the tension on the material deviating from the predetermined tension. The control system further includes means for detecting displacement of the motor relative to the frame away from its initial preset position. The control system still further includes means for adjusting the speed at which the motor rotates the shaft by an amount based upon the displacement of the motor relative to its initial position to maintain the tension of the material at substantially the predetermined tension. The adjusting means is coupled between the detecting means and the motor.
In the illustrated embodiment, the coupling means includes bearing means for permitting rotation of the shaft relative to the frame and movement resisting means coupled to the motor for resisting movement of the motor relative to the frame. The movement resisting means has a predetermined resistance related to the predetermined tension so that the motor moves relative to the frame only when the tension on the material deviates from the predetermined tension. Illustratively, the movement resisting means includes at least one spring member having a predetermined spring constant coupled between the frame and the motor.
The illustrated embodiment also includes means for measuring the radius of the roll of material, and means for coupling the measuring means to the adjusting means so that the speed at which the motor rotates the shaft is also adjusted based upon the radius of the roll of material. Illustratively, the measuring means includes an ultrasonic sensor for measuring the radius of the roll of material. The ultrasonic sensor has an output coupled to the adjusting means.
The illustrated embodiment further includes means for damping movement of the motor relative to the frame to limit the rate of movement of the motor. Illustratively, the damping means includes a piston and cylinder assembly coupled between the motor and frame for limiting the rate of movement of the motor relative to the frame. Also illustratively, the means for detecting displacement of the motor includes a differential transformer coupled to the motor. The differential transformer has an output coupled to the adjusting means.
According to another aspect of the present invention, a method is provided for maintaining a substantially constant predetermined tension on material being unwound from or wound onto a roll of material mounted onto a shaft which is driven by a motor. The method includes the steps of establishing a predetermined initial position of the motor when the material is at the predetermined tension, determining the displacement of the motor away from its initial position upon deviation of the tension on the material away from the predetermined tension, and adjusting the speed at which the motor rotates the shaft by an amount based upon the displacement of the motor relative to its initial position to maintain the tension of the material at substantially the predetermined tension.
The method also includes the steps of determining the radius of the roll of material and changing the speed of rotation of the shaft based upon the radius of the roll of material to maintain the tension on the material at substantially the Predetermined tension.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.